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Proceedings of the Nutrition Society (1999), 58, 935–944
935
The 6th International Tutorial Conference of the Research Group on Biochemistry of Exercise, a collaboration between the
Nutrition Toxicology and Environmental Research Institute Maastricht and the Nutrition Society, was held in Maastricht, The Netherlands
on 18–21 February 1999
CAB International
Symposium on ‘Metabolic aspects of human nutrition at rest and
during physical stress: recent methodological and
technical developments’
Session 4: Tracers in metabolic and nutrition research: 1
An introduction to the use of tracers in nutrition and metabolism
Michael J. Rennie
Professor Michael J. Rennie,
Department of Anatomy and Physiology, University of Dundee, Dundee DD1 4HN, UK
fax +44 (0) 1382 345514, email [email protected]
The present article is a review written at a level suitable for students and new workers to the field
of techniques in common current use for the measurement of static and dynamic features of
metabolism, especially nutritional metabolism. It covers the nature of radioactive and stableisotope tracers, the means of measuring them, and the advantages and disadvantages of their use.
The greater part of the review deals with methods for the measurement of pool sizes and metabolic
processes, with the emphasis being on protein metabolism, a field the author knows best. The
examples given are from a variety of sources, including the work of the author, but the principles
underlying the techniques are universally applicable to all metabolic investigations using tracers.
Isotopic-labelling techniques: Protein metabolism: Radioactive tracers:
Stable-isotope tracers
The word metabolism comes from Greek roots
meaning dynamic change, and it makes sense therefore
that if we want to look at the effects of altered diet
or physical activity, we should use methods which enable
us to describe the characteristics of the changes, and
in particular their rates. Isotopic-labelling techniques are
often ideal for this task. In the present review I will
discuss the use of isotopic techniques linked with detection
by mass spectrometry and liquid-scintillation counting.
There is of course a large literature on other techniques,
e.g. positron-emission tomography, magnetic-resonance
spectroscopy and near-infra-red spectroscopy, all of
which have useful applications in biomedical research,
but the topic is simply too big to discuss, and other
contributors will describe some of these techniques at this
meeting.
Nature of isotopes: stable v. radio, and methods of
measurement
What are isotopes, and which are useful for biomedical
research?
Elements which share the same place in the periodic
table are termed isotopes (Table 1). Thus, they have the
same chemistry, i.e. the same number of protons, but they
are different in their atomic mass and usually in other
physical properties also that can be used to distinguish
between them.
Common stable isotopes such as 1H, 12C, 14N and 16O
make up most of our biological environment and their
much-less-common stable companions (2H, 13C, 15N, 18O)
are by definition rare (0·02–1·1 %). However, the radioactive analogues (3H, 14C, 13N, 11O) are even more
Abbreviation: IRMS, isotope-ratio mass spectrometry.
Corresponding author: Professor Michael J. Rennie, fax +44 (0) 1382 345514, email [email protected]
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936
M. J. Rennie
Table 1. Isotopes commonly used in biological research
Common stable
Rare stable
1H
(0·02 %)
(1·1 %)
15N (0·37 %)
18O (0·04 %)
12C
14N
16O
Radioactive
2H
3H
13C
14C
13N*
11O*
* No long-lived radioisotopes of these elements.
uncommon than the rare stable isotopes, and although they
also differ in atomic mass, these masses are not constant, but
change during radioactive disintegration with the emission
of various sub-atomic particles. Thus, isotopes have the
same chemistry, but their different physical properties (e.g.
mass or radioactivity) provide the means of measurement
separately from the common stable isotopes. Modern
organic and bio-organic chemistry has given us the ability to
label almost any kind of molecule with rare, stable or radioactive isotopes, the limitations in most cases being time and
hence cost. However, in some cases such as, for example,
hydroxyproline which itself exists in four different isomeric
forms, the difficulties of synthesis are almost insuperable by
conventional means.
What are the pros and cons of the use of stable and
radioisotopes?
The major advantage of the use of stable isotopes is that they
are not a source of ionizing radiation, which means that they
are effectively safe and non-toxic, allowing studies in
children, including infants and perinates and even babies
in utero (Koletzko et al. 1997; Chien et al. 1993). Repeat
studies and long-term studies in adults are also perfectly safe
and practical, which might not be the case if radioisotopes
were used. In addition, the existence of stable isotopes of
O and N, which are of obvious biological relevance, and the
lack of any analogous long-lived radioisotope for these
elements has meant that the use of stable isotopes has grown
explosively over the past few years.
Nevertheless, there are some disadvantages. In fact the
relatively high natural background for most stable isotopes
(1 % for C and 0·3 % for N) means that at low levels of
enrichment the range over which measurements of stable
isotopes can be made is less than that for radioisotopes.
Also, most methods for detection and quantification of
stable isotopes are less sensitive than those used for radioisotopes. Furthermore, the cost of some of the isotopes (e.g.
18O) and thus of some tracer molecules made incorporating
them, even if they are available, can make some studies not
feasible. In addition, simply because of market size, the
range of molecules labelled with stable isotopes is smaller
than that of radioisotopes. The cost of synthesis of
molecules labelled with stable isotopes is always going to be
higher per unit than that for radioactive probes if the market
size is less, which is patently true for many substances.
Last, the costs of measurement instrumentation are
substantial. Not only are capital costs high, but these
instruments are complicated and difficult to use and
maintain; in addition, their use usually means that
specialized staff have to be employed to do these tasks.
These factors mean that entering into the stable-isotope
metabolic club is difficult and expensive, and requires
strength in depth, or a factor which is sadly less prevalent
than it should be, i.e. collaboration.
Before we completely leave the consideration of radioactive isotopes, we should note that in many cases, in mature
adults, the doses of radioactive tracer necessary are so low
that the safety aspects are negligible. We do ourselves a
disservice if we ignore the safe use of radioactive tracers in
situations where they are more convenient, cheaper, and
more appropriately labelled.
How do we measure isotope-labelled molecules and
implications of the methods?
and 14C emit β rays, i.e. electrons, which cause flashes of
light in a liquid scintillant which can then be detected in a
photomultiplier tube (Fig. 1). The energy spectra of 3H and
14C are different, with 14C radiation having a much higher
average energy and thus it is possible to gate the detector to
set up energy windows through which pass isotopes of
predominantly one kind or another. This process means that
we can use dual-labelling techniques with 3H and 14C.
However, there is one major disadvantage; although the
quantum efficiency of modern photomultipliers is very high
and so sensitivity is good, nevertheless the results which are
obtained after correction for quenching are measurements of
radioactivity only and not of specific radioactivity, i.e.
radioactivity per chemical mass. In order to obtain this value
we need to measure the chemical mass separately, thus there
is an inherent additional error in the calculation of the
specific labelling. Stable isotopes, and the tracer probes
containing them, differ only in their atomic or molecular
mass (i.e. the number of neutrons they contain); they can be
separated on this basis, either as native gases or as
molecules which are capable of being volatilized. Their
isotope ratio gives a direct measure of labelling or
enrichment (Fig. 2).
In cases in which the labelled material is a gas such as
13CO , C18O16O, 1H2H, or 13CO, either produced in a
2
metabolic reaction or after reduction by combustion or
pyrolysis, the isotopes in the gases can be separated after
their conversion to positively-charged molecular ions. This
conversion is usually achieved by bombardment with an
electron beam which displaces further electrons from the
sample gases, producing positively-charged molecular ions
(Fig. 2). These molecular ions can then be separated on the
3H
Fig. 1. The principle of measurement of radioactivity of radiolabelled
biomolecules.
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Tracers in metabolic and nutrition research
Fig. 2. The principle of measurement of isotope ratios for stableisotope-labelled molecules or atoms.
basis of their mass : charge ratio, either in a fixed magnetic
field produced by a permanent magnet, as in most
conventional isotope-ratio mass spectrometers (IRMS), or
occasionally by the use of a tuneable (i.e. variable)
quadrupole mass spectrometer such as in those used for
respiratory gas analysis.
Other kinds of ‘front ends’ may be used to produce
appropriate molecular gases, including elemental (Dumas)
analysers or mass spectrometers, or liquid chromatographs
coupled to a combustion or pyrolysis module. The full range
of so-called hyphenated techniques in mass spectrometry
gives us such modes as GC–combustion–IRMS,
GC–pyrolysis–IRMS, liquid chromatography–combustion–
IRMS etc.
In classical GC mass spectrometry molecules of a
relatively large size are first derivatized to make them
volatile, using a variety of techniques that add large organic
groups to the analyte (Meier-Augenstein, 1997). When these
techniques are allied with combustion mass spectrometry
(GC–combustion–IRMS), the production of large amounts
of unlabelled CO2 or CO from the adduct produces a
substantial dilution of C labelling (Yarasheski et al. 1992;
Rennie et al. 1996). The dilution factor is less of a problem
with 15N because most derivatizing agents do not add N to
the adduct formed, but there is the major difficulty of
making sure that atomospheric N is excluded from the
system.
Neither 2H nor 18O can be used in conventional
combustion systems because these produce water and CO2
as products; although CO2 can be analysed when it contains
13C, it is difficult to analyse CO containing 18O because
2
naturally-occurring CO2 of mass 44 cannot be used as an
internal standard. Pyrolysis (i.e. the partial thermal
degradation, rather than complete combustion) of organic
compounds and water containing 2H and 18O is a feasible
solution to this problem.
Continuous-flow pyrolysis IRMS offers the possibility of
measuring the labelling of H and O in microlitre samples of
water, and indeed this procedure has been used to measure
the metabolic rate of free-living bumble bees (at between
500 and 2500 ml O2/kg per min!). It furthermore offers the
possibility of detection of labelling in organic molecules at
very low levels of abundance (e.g. collagen labelled with
18O-containing amino acids and sampled over months to
years thereafter).
937
Generally, for the mass spectrometric methods utilizing
magnetic sector instruments on the one hand, or quadrupole
analysers on the other, there is a trade-off between
sample size and precision of detection limits. GC–mass
spectrometry is very sensitive in terms of sample size
(femto- to nanomoles) but requires relatively high levels of
enrichment (0·5 atom % excess) which can be measured
with a precision of approximately 0·5 atom % excess,
whereas GC–combustion–IRMS would require 1000-fold
more sample, but would allow the limit of detection to be
driven down to 0·001 atom % excess with a precision of
0·0002 atom % excess (Table 2).
Various kinds of devices which allow fine sprays of
fluids containing analytes, or which permit what is
effectively sublimation of solid samples (as a result of heat
or radiation treatment), will enable relatively large
molecules to be analysed in the gas phase. Such techniques
allow volatilization of peptides and nucleic acid fragments,
but these techniques do not normally allow the measurement
of the labelling of the individual fragments. For metal ions,
thermal-desorption and plasma-ionization techniques allow
examination of the isotope ratios of mixes of Zn, Ca, Se and
other metal ions of biological interest with great sensitivity,
accuracy and precision (Turnlund et al. 1982; Turnlund,
1991).
The dilution principle applied to determination of
isotopic labelling
The dilution principle stems directly from the Law of
Conservation of Mass. Since atoms can be neither created
nor destroyed, the total mass of isotopic tracer added to a
biological system, including that which is metabolized and
exhaled or excreted, must remain constant. Thus, if it is
known how much tracer is added to a system, sampling of
the various pools into which the tracer mixes and
measurement of its dilution in those pools will provide a
measure of the size of the pools (Fig. 3). This static dilution
technique has been used with great success in the
measurement of body composition. Models of body
composition may be constructed at atomic, molecular,
cellular and tissue levels, and at each of these levels it is
possible to use isotopes to define the size of one of the
compartments. Thus, exchangeable H, O, Na, Cl and K
(e.g. 18O, 2H, 3H, 35Cl, 42K etc.) can be used to measure
extracellular and intracellular water etc. These isotopes can
be analysed by conventional spectrometric or radiometric
methods, as appropriate. The natural radioactivity of the
Table 2. Sample size, accuracy and precision of mass spectrometers (MS): characteristics of MS methods for measurement of
enrichment of samples labelled with a stable-isotope tracer
Mode
Minimum sample size
Detection limit of
enrichment (atom %
excess)
Precision (atom %
excess)
GC–MS
Off-line IRMS GC–C–IRMS
1 pmol
1 µmol
1 nmol
0·5
0·001
0·0005
0·5
0·0005
0·0001
IRMS, isotope-ratio MS; C, combustion.
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938
M. J. Rennie
body due to the 40K left over from the ‘big bang’ produces a
very sensitive means of measuring the body cell mass
(Forbes, 1987). When bodily elements are made radioactive
as a result of small doses of neutron irradiation, a large
number of elements can be detected as a result of their own
short-term nuclear decay, including N, K, Na, C, P, Cl etc.,
and this process enables almost every compartment of the
body to be analysed with great accuracy and precision
(Forbes, 1987).
Knowing the mass of different compartments of the body
is obviously important, but in metabolic terms it is often less
interesting than knowing the rates of exchange between the
compartments or knowing the rates of metabolism of the
components within those compartments. For these purposes,
dynamic tracer dilution is required (Fig. 4).
In the simplest version of this technique, tracer is added
to a particular body pool (whose metabolism is to be
examined) at a constant rate and samples are taken at
defined intervals. The tracer will eventually equilibrate
throughout the pool, enabling the rate of flux of the tracer
and its tracee to be calculated rather simply. The movement
of metabolite through the compartment (in jargon terms the
‘flux’) is simply infusion rate : extent of labelling in a
steady-state. Of course, there are a variety of ways of
applying the tracer and a number of choices in determining
the pools to be sampled, and also many different ways
Fig. 3. The dilution principle applied to a static pool. Concentration
(c) in the sample (mg/ml) = D/(V + v), where D is the dose (mg)
in volume v (ml) and V is the volume of the pool (ml). Thus,
V = (D/C) − v.
Fig. 4. The dilution principle applied to a dynamic pool. Flux through
the pool (µmol/kg per min) = i/Ep, where i is infusion rate of the tracer
(µmol/kg per min) and Ep is the plateau enrichment.
in which the resulting information can be analysed mathematically and statistically (Waterlow, 1984; Cobelli et al.
1987; Wolfe, 1992).
The constant-infusion method (Fig. 5(a)) is the simplest
conceptually, and as long as sufficient samples are taken it
provides information on the degree to which equilibrium (or
at least a metabolic steady-state) has been achieved,
signalled as the relative constancy of labelling of the
metabolic pool in question. An alternative approach is to
deliver the tracer as a bolus to one of a number of accessible
metabolic compartments (Fig. 5(b)). This approach
obviously provides dynamic information on those pools that
can be sampled directly or, in the case of breath and urine, of
the whole body treated as a single pool. However, by using a
spike input of tracer into one metabolic pool together with
sampling from another interconnected pool, it is possible to
gain information about the movement of substances and
interconversion in metabolic pools that are otherwise
inaccessible. Thus, injection of tracer into plasma and
sampling from plasma nevertheless may provide
information about other pools such as extracellular and
intracellular compartments. In order to obtain reliable
information the input function must be well defined and
should, if possible, be instantaneous (or at least occurring
over a known time period), mixing should be rapid (or at
least over a defined period), a steady-state must be assumed,
and rapid sampling must be possible in at least one of the
pools. Some extremely sophisticated methods of analysis
have been applied, together with imaginative sampling
protocols, to enable us to gain large amounts of information
about the metabolic interchange turnover and compartmental size for a variety of metabolic processes. Among the
major difficulties of compartmental analysis, however,
are that often the pools are only notional, being the
conglomerate of groups of pools which exist in a variety of
different tissues (e.g. the extracellular space); furthermore, it
is often very difficult to obtain dynamic information for
more than four interconnected pools, especially as the
labelling of the primary pool from which the sample is taken
starts to approach zero, and often large numbers of laborious
experiments have to be conducted to examine the behaviour
of secondary metabolites. This situation occurs particularly
when we are examining changes in patterns of nutrient
intake or composition, physical activity and disease state.
If compartmental modelling is avoided, then a single
bolus input combined with measurement of the tracer in a
metabolic pool or total production of some metabolite of the
tracer can give large amounts of useful information. The
simplest example of this procedure is the use of the doublylabelled water method for measuring energy expenditure, in
which both H and O in water are labelled. The dose divided
by the tracer labelling at time zero (extrapolated from the
results of analyses of later samples) provides a measure of
the total body water and thus of lean body mass (Wolfe,
1992). As long as it is assumed that the rate of loss of 2H and
18O labelling from the body water pool each follow a single
exponential decay (of the form Et = E0 e-kt), where Et and E0
are the enrichments at zero time and time t and k is a
constant, then the difference in the rates of loss of 2H and
18O will provide a measure of CO production; given an
2
assumed RQ of 0·85, this measurement provides a measure
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Tracers in metabolic and nutrition research
939
Fig. 5. (a) The constant-infusion method and (b) the bolus-injection method, with compartmental (pool) analysis, for measuring isotopic labelling
of a metabolic pool. (&), Plasma; (%), (k), extracellular (ECS) and intracellular (ICS) space.
of O2 consumption and thus of energy expenditure. This
topic is explored in more detail by Klaas Westerterp (1999).
If a metabolic end product is measured (e.g. CO2 in the
breath or urinary urea and/or NH3 after administration of,
for example, [13C]leucine or [15N]glycine), then the flux
through the pool is equal to the dose divided by the area
under the endproduct curve, assuming that the tracer has as
much chance of getting into the metabolic endproduct as
does the tracee. This factor is not usually a problem in the
case of relatively simple metabolic processes such as the
production of CO2 from glucose, but when a particular
amino acid (e.g. [15N]glycine) is used as being representative of all other amino acids, then unless its metabolism
truly is representative (which in the case of glycine it almost
certainly is not; Matthews et al. 1981) difficulties of interpretation may arise.
Fewer difficulties arise if the probe used is metabolized
rapidly to an endproduct via a metabolic process which may
often be rate limiting. This was the case in a study we
conducted of a child with Crohn’s disease investigated
before and after treatment with steroids, when the difference
in the appearance of 13CO2 from orally-administered
[13C]leucine was very apparent (Fig. 6; Thomas et al. 1992).
In this case the process of interest was absorption via the
gut. In general such breath tests can give a substantial
amount of information concerning gastric emptying,
oro-caecal transit time, the presence or not of Helicobacter
pylori infection, gut bacterial overgrowth, the extent of liver
function, fat digestion, and absorption, protein digestion,
and energy expenditure. All these processes use 13C
exhalation in the breath CO2 as a probe for the physiological
function of interest.
Fig. 6. The effect of treatment on labelled carbon dioxide in expired
breath after ingestion of [1-13C]leucine in a child with Crohn’s
disease. The difference presumably represents more efficient small
intestinal absorption. APE, atom % excess. Flux = dose/∫E13CO2,
where E13CO2 is 13C enrichment of CO2. Mean residence time = pool
size/flux. It is difficult here to estimate pool size as dose/Eo, where Eo
is the enrichment at zero time. (From Thomas et al. 1992.)
Intermediary metabolism and protein turnover
When more extensive information is required about interrelated metabolic processes (e.g. in the case of leucine
which may be incorporated into body protein, or appear
from body protein in the processes of protein turnover, or
which may be transaminated to its α-keto acid, which is
thereupon oxidized), a much greater range of analytes will
need to be sampled in order to provide such information. In
my opinion such experiments are easiest to carry out under
conditions in which primed constant infusions of tracer
have been applied, in which case it is possible to make
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940
M. J. Rennie
measurements in the steady-state of a number of integrated
processes. A good example is the measurement of the
whole-body leucine rate of appearance and oxidation which,
when subtracted, provides information on whole-body
leucine disappearance into protein, i.e. protein synthesis.
(Matthews et al. 1981; Fig. 5(a)). Individual tissue protein
synthesis can be calculated using the same tracer administration protocol, while in addition measuring the rate of
increase in enrichment of the protein sampled by biopsy
(Fig. 7; Smith & Rennie, 1990).
Methods for measurement of macromolecular turnover
This topic has been touched on earlier, where it was shown
that tissue protein synthetic rate can be determined by means
of the change in enrichment with time divided by the
average extent of labelling in the precursor pool. (Fig. 7).
An alternative method is to measure the rate of dilution of
the tracer across an organ (ideally assuming that the tracee is
Fig. 7. Typical pattern of labelling of plasma free leucine and tissue
protein leucine during a primed constant infusion of [1-13C]leucine.
Tissue protein synthesis rate (%/h) = ∆Etiss/∫Ep × 1/t, where Ep is
plateau enrichment, ∆Etiss is change in tissue enrichment and t is
time.
not produced metabolically (as distinct from via turnover)
by that organ, which is often the case) and then taking the
measurement of the tracer dilution as a measure of the
breakdown of a macromolecule to its monomer. This
method would work very well for, for example, a measurement of protein breakdown as dilution of tracer
phenylalanine via release of unlabelled phenylalanine. The
analysis in the case of a study of muscle is facilitated by the
fact that phenylalanine is an essential amino acid (i.e. is not
synthesized de novo in mammals) and shows no intermediary metabolism. Thus, the rate of dynamic dilution is
equivalent to the rate of protein breakdown, and the rate of
protein synthesis is therefore simply the input minus output
of phenylalanine, i.e. the net phenylalanine balance across
the tissue or organ under consideration, plus protein
breakdown (Bennet et al. 1990; Rennie et al. 1990). A major
and very useful refinement of the method has been to model
not only rates of protein metabolism but also transport into
and out of tissues (Biolo et al. 1992; Fig. 8).
The caveats which Ian Macdonald (1999) discussed for
the normal measurement of arterio–venous flux in organs
are doubly important when it comes to the measurement of
tracer exchange, i.e. there must be a steady-state of concentrations (particularly of tracee and its tracer) and the flow
and transit time of the measured substances across the organ
must be less than the intervals of sampling. However, in the
case of tracers another condition needs to be fulfilled, and
that is that the pool in which the greatest extent of dilution is
taking place must be sampled in order for the model to be
applied accurately. In many cases the venous output has
been used, but actually this, because of shunting between the
arterial and venous blood, provides an underestimate of the
extent of dilution of label. Ideally it should be sampled from
the extracellular space (e.g. by means of the dialysis
technique; see Arner, 1999) or even in the intracellular free
amino acid pool, if this were truly possible. Usually workers
in the field make do with either the venous plasma labelling
or that of the tissue free amino acid, which is mainly
intracellular.
An alternate way to measure protein breakdown,
applicable to any tissue for which the tissue free amino acid
pool can be sampled, is the fractional breakdown rate
method (Fig. 9; Zhang et al. 1996). The method depends on
measuring the dilution in the venous blood draining a tissue,
or in the tissue free pool, of the tracer after its delivery has
been stopped. This dilution is a measure of the appearance
of unlabelled amino acid from tissue protein, i.e. protein
breakdown.
The precursor–postcursor problem
Fig. 8. Arterio–venous tracer exchange model which can give
values for transport of amino acids into and out of muscle or other
tissues, as well as values of disappearance into protein (synthesis)
and appearance from protein (breakdown). (m), Tracer; (l), tracee.
Balance = Ca − Cv, where Ca and Cv are the arterial and venous concentrations respectively. Breakdown = 1 − Ea/Etiss × flow × Ca, where
Ea and Etiss are the arterial and tissue enrichments respectively.
Synthesis = balance + breakdown. Using this method it is also
possible to measure the tracee, and tracee transport in and out of the
tissue.
The problem of adequately sampling a particular labelled
pool the results of which will be used to calculate a
particular metabolic function, is endemic in the study of
metabolic processes using tracers. In the study of the
synthesis of macromolecules it is known as the precursor
problem, but in fact it could well be called the postcursor
problem (if that is not too ugly a neologism) when applied to
the measurement of the extent of dilution of a tracer in the
free pool as the result of breakdown of the tracer. If a tracer
amino acid is added in such a way (e.g. by constant infusion)
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Tracers in metabolic and nutrition research
941
Fig. 9. The fractional breakdown rate method. (a) Tracer is infused at a constant rate in order to measure synthesis rate and (b) dilution is
measured after infusion is stopped as a measure of breakdown. (l), Tracee; (m), tracer; S, synthesis; B, breakdown; ↑, sampling times. (Zhang
et al. 1996.)
Fig. 10. Notional values for tracer : tracee (% enrichment) for a nonmetabolizable tracer amino acid in various free pools in a tissue such
as muscle or liver. Where there is de novo synthesis and metabolic
chanelling the tRNA pool may be substantially less than that of the
intracellular space (ICS; e.g. as for proline in the skin). A, arterial
blood; V, venous blood; ECS, extracellular space.
that it achieves a particular labelling in the arterial blood,
dilution of the tracer results in a much lower enrichment of
other pools (Fig. 10).
The tracer may be diluted to an unknown extent in some
pools, such as the lysosome and proteosome. The difficulty
is that in order to measure the true rate of protein synthesis
or the true rate of protein breakdown, it is imperative to
characterize the labelling in the appropriate pool. The
situation is no different when it comes to measurement of
synthesis or breakdown of, for example, glycogen (in which
the appropriate precursor pool would be that of glucose-6phosphate) or triacylglycerol (in which case it would be
useful to know the extent of the labelling of free fatty acids
and newly-synthesized glycerol-3-phosphate), or of RNA
(in which case it would be important to know the amount of
labelling of, for example, ribose, or of a free purine or
pyrimidine, or of the nucleotide phosphate).
The importance of getting the value right is demonstrated
by results of an experiment my colleagues and I carried out
in which we presented leucine or valine labelled with 13C
either intragastrically or intravenously, and observed that
when we measured the labelling of the free tracee in the
human duodenal mucosa mixed-tissue water there was a
twofold difference in the tracer : tracee value (Nakshabendi
et al. 1995; Fig. 11). However, there was also a twofold
difference in the labelling of duodenal mucosa protein, so
that when protein synthesis was calculated on the basis of
the labelling in the free pool of the tissue in each case
(irrespective of whether the tracer had been administered
intragastrically or intravenously, or whether it was leucine
or valine) the rate of protein synthesis was found to be
approximately 2·5 %/h. In this case we did not measure the
labelling of intracellular tRNA, but where we have done so
(e.g. in the pig stomach) we have shown that leucyl tRNA
reaches to a value which is approximately that of the gastric
mucosal intracellular pre-water labelling and also of the
plasma α-ketoisocaproate labelling (Watt et al. 1992). In
these circumstances it is therefore possible to use the plasma
α-ketoisocaproate labelling to provide an indication of the
intracellular labelling, as we have done most successfully
for muscle in which the intracellular free pool, the leucyl
tRNA and the plasma α-ketoisocaproate labelling give
similar results (Watt et al. 1991). In the case illustrated in
Fig. 11, the protein synthetic rates estimated from the
protein labelling and the intracellular pools were identical
for both tracers used and both routes of administration.
It is also possible to measure the rate of synthesis of
secreted proteins, e.g. pepsin as the rate of incorporation of
13C into pepsin protein isolated from gastric juices (Corbett
et al. 1995). Other workers have used the same techniques to
measure pancreatic protein synthesis or the synthesis of
export proteins from liver such as albumin, apolipoprotein
B100 etc. (Collins et al. 1992; Bennet et al. 1993).
However, in some circumstances, for example if we wanted
to measure the rate of labelling of a macromolecule
synthesized in a relatively inaccessible pool, it would be
very difficult to use this kind of methodology. For example,
in bone and skin, collagen is made by cells which are
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942
M. J. Rennie
Fig. 11. Differences in 13C labelling in (a) the free and (b) the protein-bound amino acid pools when leucine (leu) or valine (val) labelled with 13C
were administered either intravenously (iv; ]) or intragastrically (ig; $). Values are means with their standard errors represented by vertical bars
for n 6-8 determinations. (From Nakshabendi et al. 1995.)
embedded in an extracellular matrix which almost certainly
does not, for example, participate in the transamination of
leucine to α-ketoisocaproate at rates sufficiently high to
give us confidence that plasma α-ketoisocaproate provides a
good measure of intracellular leucine labelling, and in these
circumstances application of the so-called flooding-dose
technique might be particularly useful (Scrimgeour et al.
1993; Rennie et al. 1996) The idea here is to equilibrate the
free pools by providing so much (in a chemical sense) of the
labelled amino acid that an equilibrium is forced.
Constant infusion or flooding dose?
The so-called flooding dose approach was first used by
scientists interested in the measurement of protein turnover
in tissue culture; it was first applied to living mammals by
McNurlan & Garlick (1980), who used it to study protein
turnover in tissues of the rat. When it was applied to adult
human subjects the tracer used was leucine labelled with
13C; later, 2H-labelled tracer was used because of the ability
of GC–mass spectrometry to measure [2H]phenylalanine in
hydrolysates of protein into which the tracer had been
incorporated (McNurlan et al. 1991; Calder et al. 1993).
One of the difficulties with this method is that, although it
appears to be perfectly acceptable for measuring decreases
in the rate of protein synthesis, it seems to have very little
dynamic range in detecting increases above the normal
physiological value, at least in muscle. In our group we were
particularly worried, when looking at values first obtained,
that the method appeared not to be able to detect the effects
of feeding on protein synthesis, and therefore we hypothesized that the flooding technique itself apparently
stimulated protein synthesis. In a long series of experiments
we have now shown that when muscle protein synthesis is
measured in human subjects using a flooding dose of an
essential amino acid there is an apparent stimulation (which
may, in fact, be a true stimulation of muscle protein
synthesis), but when nonessential amino acids such as
glycine, proline, serine or arginine are used there is no such
apparent stimulation (Smith et al. 1992a,b,c, 1998; Fig. 12).
This fascinating result will not be discussed further here,
but I have to say that it strongly suggests that there is some
control by amino acid availability and type on the rate of
lean mass accretion in the body. In addition, it may turn out
to be very useful that proline does not invoke this effect,
since it has been possible to measure the rate of bone and
Fig. 12. Effect of flooding dose of various amino acids, both
essential and nonessential, on muscle protein synthesis. ((), Tracer
alone; (]), leucine (leu) flood; (\), glycine flood; (%), proline flood;
(&), serine flood; ($), arginine flood. Val, valine; ala, alanine; FSR,
fractional synthetic rate. Values are means with their standard errors
represented by vertical bars for n 6-8 determinations.
skin collagen synthesis using the flooding-dose protocol
(Scrimgeour et al. 1993).
Measurement of nucleic acid turnover using
stable-isotope techniques
The measurement of the turnover of RNA and DNA is of
great interest to those of us concerned with regulation of
body metabolism. There have been a number of attempts to
measure the breakdown of RNA and DNA using specific
non-reutilizable metabolites such as dimethylguanosine and
pseudouracil (Marway et al. 1996), but the measurement of
nucleic acid synthesis in vivo has been much more difficult.
There are a large number of labelling choices for the study
of the synthesis of RNA and DNA, since molecules which
could be incorporated include phosphate, ribose, glycine,
glutamine and, of course, the bases themselves (Berthold
et al. 1995; Boza et al. 1996; Perez & Reeds, 1998). In
addition, some of the constituents are methylated from
methionine, which increases the number of possibilities of
labelling. Unfortunately, the situation is markedly complicated by the existence of salvage pathways which recycle
the bases into nucleic acids. Furthermore, the labelling of
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Tracers in metabolic and nutrition research
the bases with adenosine and guanosine is difficult because
of the substantial pool sizes.
In another approach, Macallan and co-workers
(Hellerstein et al. 1999) used glucose as a precursor of
ribose to measure leucocyte labelling in vivo. Unfortunately,
because of the relatively small production of ribose from
glucose they had to use glucose at 20 atom % in order to
achieve sufficient labelling. This approach is probably
inordinately expensive as a means of routinely measuring
nucleic acid turnover. Obviously, if these techniques can be
made to work with mRNA it will be possible, at long last, to
make molecular biology quantitative!
Summary
The techniques outlined here depend, to a large extent, on
the availability of instrumentation which requires
substantial capital expenditure, and on the availability of
trained scientists and technicians who have particular
expertise in maintaining and operating modern mass
spectrometers. Although the user-friendliness of the
technology is improving year-by-year, and increasing
numbers of stable-isotope-labelled compounds are
becoming become available, so that almost any metabolic
problem could theoretically be tackled with the equipment
currently on offer, for most people starting in the field there
is a very substantial barrier. In my view the best way we can
overcome this problem is by collaborating between groups
so that individuals with particular expertise in, for example
vitamin metabolism, may co-operate with others with mass
spectrometry expertise in order to make kinetic measurements of vitamin turnover. To date, the nutritional and
metabolic community has been very poor at this kind of
collaboration, but without it, it will be much more difficult
to tackle the many interesting and important questions
which currently confront us.
Acknowledgements
Personal work described in this paper was supported by the
UK Medical Research Council, The Wellcome Trust, The
University of Dundee, Action Research, and the Chief
Scientist’s Biomedical Research Committee of the Scottish
Home & Health Department. This paper was written during
the tenure of a sabbatical at Shriners Burns Institute,
Galveston, Texas, USA.
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